1. Field of the Invention
The present invention relates to a dot analysis apparatus that analyzes a regular arrangement of dots forming a dot image, a dot analysis program storage medium, and a dot analysis method.
2. Description of the Related Art
Most images printed on recording media, such as paper, are dot images represented by dots. In general, such prints are produced by converting a manuscript into a dot image, printing the dot image onto a film, making a machine plate from the film, and then installing the machine plate in a printing machine. It is common that users have only the print produced by such a process or the film used in plate making and, thus, produce a print by electronically reading the print or film with a color scanner to produce electronic data, making a machine plate for printing based on the electronic data, and then installing the machine plate in a printing machine. In such a case, the printing machine used this time may have a resolution slightly different from that of the printing machine used to produce the print kept as an original or the printing machine expected to be used to produce a print from the machine plate made from the film kept as an original. If the print is produced without being aware of the resolution difference, the resulting image may be significantly degraded, for example, moiré fringes may appear in the image. To address such a problem, there has been known a processing referred to as descreening, which is a processing that converts electronic data obtained by a color scanner or the like from a print or film kept as an original into electronic data representing a smoothed image with a smoothed dot structure using a conversion filter for smoothing fine structures of an image.
In the descreening processing, image data that represents a dot image by a two-dimensional array of pixels assigned with numeric data is processed as described below, for example.
First, a conversion filter that converts image data representing a dot image into image data representing a smoothed image with the dot structure smoothed is prepared. The conversion filter is mathematically represented by a two-dimensional matrix of a predetermined size.
Then, a data processing described below is performed on each of plural pixels forming the image to be processed. First, the matrix representing the conversion filter described above is overlaid on the array of pixels centered on the current target pixel. Then, the values of the elements of the matrix are multiplied by the numeric data of their respective overlapping pixels, and the sum of the products obtained for all the elements of the matrix is calculated. Then, the original numeric data of the target pixel is replaced with the sum of the products.
Performing such a data processing on all the pixels forming the image will be referred to as filtering hereinafter. In the descreening processing, such filtering is performed on the image data one or more times, thereby smoothing the dot structure of the dot image.
In order to accomplish such a descreening efficiently, it is desired that an interval between regularly arranged dots of the dot image and a direction of the regular arrangement of dots of the dot image are grasped as precisely as possible before processing, and an appropriate conversion filter for the interval and direction is prepared.
FIG. 15 is a schematic diagram showing an example of an appropriate conversion filter for an interval between regularly arranged dots of a dot image and a direction of regular arrangement of dots of the dot image.
FIG. 15 shows plural dots P and a conversion filter F overlapping with a dot image formed by the dots P. In the conversion filter F, the elements lie within a square region Q whose side is equal to an interval L between the dots P and which is oriented in the same direction as the arrangement of the dots P are assigned with a constant value other than 0, and the elements outside the region Q are assigned with a value of 0. The filtering described above is performed on the image by shifting the conversion filter represented by such a matrix in the direction of the arrow D in the drawing, and thus, the dot structure of the dot image is efficiently smoothed.
In order to accomplish such an efficient descreening processing, there has been proposed a descreening device that can perform the Fourier transform on the image data representing a dot image to be processed to determine the frequency characteristic of the spatial structure of the image, represent the frequency characteristic by the polar coordinates and calculate an interval between dots of the dot image and a direction of arrangement of the dots from the spatial frequency and angle corresponding to the peak of the frequency characteristic represented by the polar coordinates (for example, see Japanese Patent Laid-Open No. 11-155067, paragraphs [0027] to [0059] and FIG. 1).
However, the calculation of the interval between dots and the direction of arrangement of the dots performed by the descreening device disclosed in Japanese Patent Laid-Open No. 11-155067 (paragraphs [0027] to [0059] and FIG. 1) has a problem that the frequency characteristic determined by performing the Fourier transform on the image data contains much noise, such as undesired frequency components irrelevant to the dot structure, so that the calculation precision is lowered because such noise cannot be completely eliminated by a band pass filter or the like.
Here, the calculation of the interval between dots and the direction of arrangement of dots can be applied not only to the descreening device but also to other processings, such as scaling up and down of a dot image, for example. Therefore, not only the descreening device but also such other processings have the same problem of the lowering of the calculation precision.